1. Field
One or more aspects of embodiments according to the present invention relate to a system for synthetic array sensing, and more particularly to a system for maintaining ping-to-ping alignment of phase centers in a synthetic array sonar.
2. Description of Related Art
In a system using a directive sensor, be it a RADAR, a sonar or a telescope, the ability of the sensor to collect data from a given direction and to discriminate against data arriving from a different direction is a function of the effective aperture of the receiver. The greater the effective size of a receiver, the more tightly a directional beam can be formed.
In certain applications, there are limits on the physical size of a receiver, set by the mechanical constraints of the platform to which the sensor is to be attached. For example, it may not be practical to fit a long radar antenna, e.g., an array of receiving elements, on the side of a relatively short aircraft fuselage. Synthetic aperture beam-forming mitigates the limitations related to this constraint, allowing higher resolution images to be obtained than are possible with a single physical antenna. The process of synthetic aperture beam-forming may be used in the field of RADAR and sonar processing. In a synthetic aperture sonar (SAS) system, or a synthetic aperture RADAR (SAR), an antenna is moved through the water or air, while observing a static scene. The data from successive transmitted pulses, or “pings,” is combined coherently to create an image equivalent to that which could have been obtained from a single array with an aperture the same as the distance traveled over the period of coherent summation.
To coherently combine data from successive pings, it is necessary to have an accurate estimate of the position and attitude of the array at each ping, and to correctly map the trajectory of the sensor during the acquisition cycle. In the case of SAR, navigational aids, such as a global positioning system (GPS) signal, together with inertial sensors, typically derived from rate gyro and accelerometer data combined in a Kalman filter, provide an aided inertial navigation solution with the degree of precision necessary to allow coherent combination. Underwater, in the case of SAS, GPS information is not available, and the comparatively slow wave speed, when compared to the vehicle speed, means that data obtainable from even an aided inertial navigation system is not good enough to provide the precise (sub wavelength) navigation required for coherent combination.
In an SAS system, displaced phase center antenna (DPCA) processing, which may also be referred to as redundant phase center (RPC) processing, may be used to provide high accuracy navigation. In such a system, contiguous sets of receiving elements may be grouped into staves, and the signals from all of the receiving elements in each stave summed into respective stave outputs, for subsequent processing of the returns from each ping. Pings may be timed, based on the forward motion of the vehicle, to cause the phase center for one stave, i.e., the mid-point between that stave and the transmitter, for one ping, to be in the same (nominal) position as the phase center for another stave in the array on the next ping. This may require adjusting the interval between pings as the vehicle velocity changes, a requirement that can be burdensome if multiple sensors on the vehicle must also be synchronized to avoid interference, e.g., if the other sensors require that the repetition rate of the pings be constant. Thus, there is a need for a system and method for achieving phase center alignment without constraining the interval between pings, and, in particular, compatible with a fixed repetition rate.